Virtual Terminal Sharing

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When you introduce virtual terminals to your network, you introduce additional VTAM® resources, because virtual terminals use CSA and other resources. With the proper virtual terminal sharing techniques, you can cut down on VTAM resource requirements for your virtual terminals.

There are two types of virtual terminal sharing:

  • ACB sharing, in which an OPEN-ACB can have OPEN-DESTs to sessions with more than one VTAM application
  • parallel sessions, in which an OPEN-ACB can have more than one OPEN-DEST to the same VTAM application

Virtual devices can be shared across more than one virtual terminal pool and can also be shared among VTAM applications. For example, the same virtual terminal can be used for a virtual session with TSO, IMS, CICS®, and NetView®, for either a single user session or a combination of user sessions. However, the virtual terminal cannot be in session with the same application more than once, unless both the virtual terminal and the application can support parallel sessions.

Parallel sessions can be established only if
  • the VTAM application APPL can support parallel sessions and has coded in SYS1.VTAMLST(newname)
  • the virtual device has PARSESS=YES coded in SYS1.VTAMLST(newname)
  • the virtual terminal pool definition (the VSM DEFINE command) specifies the PARALLEL parameter

CL/SuperSession, OMEGAMON®, and TSO can support parallel sessions.

Unless you use parallel sessions, each application requires as many virtual devices as the maximum number of concurrent users.

Applications (such as IIN) that use more than one CLSDST PASS to pass sessions inhibit virtual terminal sharing. The virtual terminal treats a passed session as a session termination followed by a session initiation, and receives no information to correlate the two sessions. If two such session passes are in transit at the same time, the virtual terminal cannot determine which of them was completed first.

The virtual terminal cannot control session passing. To avoid crossed sessions, CL/SuperSession requires that you specify in advance which sessions will be passed (CLSDST PASS) more than once. Terminals that support such a session are not selected to support another, and should be assigned to a pool with the DEDICATE parameter.

Applications that pass a terminal (CLSDST PASS) exactly once and do so quickly (for example, NetView) only temporarily inhibit sharing, while the pass is occurring. Once a session has been passed, it is not passed again, and the virtual terminal can support another such session. This type of application should be assigned to a pool with the PASS parameter.

If a pool does not contain enough virtual terminals for your needs, you may see this message on your screen: 


Session establishment failed because the Virtual Session Manager could not 
allocate resources.

In this case, be sure to increase your pool size. Otherwise, your users may not be able to establish the sessions they need.